1. Field of the Invention
The present invention relates to a cardiac stimulator of the type having a pulse generator and an electrode system which includes an atrial electrode, arranged in an atrium of a heart, and a ventricular electrode, arranged in a ventricle of the heart, whereby stimulation pulses, generated by the pulse generator are supplied to the atrium via the artial electrode or to the ventricle via the ventricular electrode for stimulating a heart response.
2. Description of the Prior Art
In a dual chamber system of the above-described type, the atrium and ventricle can be stimulated in a synchronous sequence which emulates a natural heart cycle in a healthy heart. In order to prevent the release of needless stimulation pulses to the heart, the atrium and the ventricle can be sensed for spontaneous (natural) activity. For example, if the ventricle is sensed for spontaneous activity, ventricular stimulation pulses are inhibited when a spontaneous ventricular event (ventricular systole) is sensed. Such a system is designated DVI. The DVI-system is further refined the if the atrium is also sensed for spontaneous activity. A sensed spontaneous atrial event (atrial systole) would then inhibit emission of an atrial stimulation pulse. This refined system is known under the designation DDI. A heart stimulator is designated DDD if it is devised for both triggering and inhibiting according to spontaneous activity sensed in both the atrium and the ventricle.
European Application 0 308 536 describes a heart stimulator which can stimulate and sense both in the atrium and ventricle. This known cardiac stimulator has two electrodes in the atrium (atrial electrodes), two electrodes in the ventricle (ventricular electrodes) and an indifferent electrode, consisting of the cardiac stimulator's capsule. Stimulation and sensing of both the atrium and the ventricle can be achieved either between an electrode and the capsule or between the two electrodes in the respective chamber, i.e. atrial stimulation and sensing between one atrial electrode and the capsule or between two atrial electrodes, and ventricular stimulation and sensing between one ventricular electrode and the capsule or between two ventricular electrodes.
The electrodes are implanted in the heart on electrode leads. For this known cardiac stimulator, a bipolar electrode lead can be implanted in the atrium, and a bipolar electrode lead can be implanted in the ventricle. The electrode emitting the stimulation pulses must be in contact with heart tissue in order to stimulate that tissue. A sensing electrode, however, does not need to be in contact with heart tissue, since blood in the heart conducts current better than the tissue itself. It is nonetheless preferable to have even the sensing electrode in contact with heart tissue, since this will result in more distinct signals. This is because an electrode which is not in direct contact with tissue picks up signals from a large area of tissue. In addition, interference signals are conducted better in blood than in tissue and thus affect the measurement signal to a larger degree.
As an alternative to two bipolar electrode leads, a quadripolar electrode lead can be used, provided it is constructed so at least one of the two electrodes located in the atrium is in contact with heart tissue for stimulating same. The advantage of a quadripolar electrode lead is that it facilitates implantation, since only one electrode lead has to be introduced into the heart. The disadvantage of a quadripolar electrode lead is that it is thicker and stiffer than a bipolar electrode lead.
In European Application 0 596 319 (published after the filing of the prior Swedish application on which the present application is based), a cardiac stimulator which has an electrode system for sensing atrial and ventricular heart events is described. The electrode system has one atrial electrode, one ventricular electrode and an indifferent electrode, consisting of the heart stimulator's capsule. Ventricular events are sensed in the same way as in the above-described known cardiac stimulator, i.e. between the ventricular-electrode and the indifferent electrode. Spontaneous atrial activity is sensed between the atrial electrode and the ventricular electrode. With this design, the number of conductors in the electrode lead can be reduced compared to when a separate sensing electrode is used, as in the above-described known heart stimulator.
In order to operate as effectively as possible in the DDD mode, the cardiac stimulator should automatically ascertain whether emitted stimulation pulses result in any heart events, i.e. evoked responses. If provided with such a function, the heart stimulator will have an ability to automatically set the stimulation amplitude at a value which is as close to the heart's stimulation threshold value as possible, thereby saving energy. This function is known as autocapture.
One problem associated with the autocapture function is to distinguish between signals having different origins in the heart without the need for excessively comprehensive electronics or circuit logic. Cardiac stimulators are designed so as not to impede or disturb the patient receiving the heart stimulator. Therefore, the volume and weight of the heart stimulator are restricted as much as possible, and this naturally limits the possibility of incorporating comprehensive electronics in the stimulator.